Various methods are known for the extra-corporeal treatment of blood in which the patient's blood flows through a blood treating unit in an extra-corporeal blood circulatory system. One of the chief complications of the extra-corporeal treatment of blood, such for example as hemodialysis or hemofiltration, is the possibility of air penetrating into the extra-corporeal blood circulatory system.
To separate entrained air bubbles from the blood, drip chambers are arranged in the venous segment of the extra-corporeal blood circulatory system. The known drip chambers are highly reliable in trapping the air bubbles. Nevertheless, there is a risk of air bubbles being infused into the patient intravenously. For further safety, parts of the blood treating apparatus have air detectors, for whose reliability in operation very stringent requirements are set. Air detectors must be capable of detecting large and small air bubbles with high reliability. If a large air bubble is detected, the treatment is immediately suspended and an alarm given, although a certain number of smaller air bubbles can be tolerated. However, the total volume of air contained in the blood must not exceed a preset limiting value.
EP 1182452 A2 describes an arrangement for detecting air bubbles in flowing liquids that is based on ultrasonic measurement. The air bubbles are detected from the attenuation of ultrasonic signals traveling through a measurement gap. The amount of attenuation is a measure of the size of the air bubbles.
The monitoring arrangement disclosed in EP 1182452 has an ultrasonic emitter for coupling the ultrasonic signals into the flowing liquid at a preset level and an ultrasonic receiver for receiving the ultrasonic signals emerging from the flowing liquid. The output signal from the ultrasonic receiver is compared with a preset limiting value. If the ultrasonic signal is below the limiting value it is assumed that there is a large air bubble present. If the ultrasonic signal is equal to or above the limiting value it is assumed that there is a small air bubble present.
When there is a large air bubble, the monitoring arrangement at once gives an alarm. The occurrence of smaller air bubbles, on the other hand, does not immediately result in an alarm. An alarm is only given when the air situated in the flowing liquid exceeds a critical total volume. The volume of air contained in the liquid is calculated from the number of small air bubbles and the flow-rate of the flowing liquid. The small air bubbles that are detected by the ultrasonic measurement are counted for this purpose. The monitoring arrangement known from EP 1182452 A2 is intended particularly for monitoring the extra-corporeal blood circulatory system of a blood treating apparatus.
U.S. Pat. No. 4,651,555 describes a method of monitoring a flowing liquid for the presence of particles or gas bubbles, in which sound signals are coupled into the flow of liquid and the sound signals emerging from the liquid are received. The sound signals received are compared with a limiting value, and it is concluded that an air bubble is present if the sound signals received are lower than the limiting value. The limiting value with which the sound signals are compared is determined on the basis of a mean value which is formed from the sound signals received.
A monitoring method for flowing fluids which is based on the analysis of the damping of sound signals which travel across a measuring gap is also known from U.S. Pat. No. 4,487,601.
EP 1466637 A2 describes an arrangement for detecting air bubbles on the basis of an ultrasonic measurement, in which the envelope of the signal received is analyzed to allow the volume of the air bubbles to be determined. If the total volume of the air bubbles exceeds a preset limiting value within a given period of time, an alarm is triggered.